1. Field of the Invention
The present invention relates to an optical subassembly installed in an optical transceiver, in particular, the invention relates to a receiver optical subassembly.
2. Related Prior Art
The conventional receiver optical subassembly (ROSA) is configured to install a temperature sensing device such as a thermistor and to adjust a bias voltage applied to a light-sensing device based on a temperature sensed by the thermistor. Especially, when an avalanche photodiode (APD) is used as the light-sensing device, it is required to adjust the bias voltage applied thereto based on an ambient temperature because the multiplication factor of the APD strongly depends on the temperature of the device. Japanese patent application published as JP-H05-243588A has disclosed an optical module installing the APD and a heat sink that serves as a bottom plate of the module package. On the bottom plate is installed a Peltier device, and the APD is mounted on the cooled plate of the Peltier device with a thermistor. The thermistor senses the temperature of the cooled plate to adjust the magnitude of the driving current supplied to the Peltier device. Thus, the temperature of the APD can be controlled.
When the optical subassembly is applied to an optical communication system whose bit rate is relatively low, for instance, slower than 2.5 Gbps, the optical subassembly is unnecessary to sense the temperature of the light-receiving device with the thermistor. However, if the bit rate of the optical communication system reaches 10 Gbps, or greater than 10 Gbps, it is typically adapted so that a box-shaped package, referred to as a butterfly package, is configured to mount a wiring substrate made of ceramic material on a bottom plate made of metal such as Kovar or copper tungsten (CuW), and to transmit signals on the interconnections that satisfies the impedance matching condition on the ceramic substrate.
Such a butterfly package generally provides enough space to install components therein compared with a cylindrical package, which is referred to as a co-axial package and is widely applied to the optical subassembly, and to form lead terminals in the side or bottom of the package to output the sensed signals from the thermistor. The thermistor installed in the package senses the case temperature of the package, which corresponds to the operating temperature of the APD to adjust the bias voltage applied thereto. Various characteristics of the APD, such as the breakdown voltage and the carrier multiplication factor, strongly depend on the temperature and, when the operating or ambient temperature of the APD varies, the bias voltage must be adjusted depending on the temperature.
However, the butterfly package has a fatal disadvantage that the price thereof is generally expensive compared to the co-axial package. Because the co-axial package is widely used not only in the optical communication system but also various consumer products, the price of the co-axial package is far cheaper than that of the butterfly package. In other words, this is the subject confronted with the optical subassembly.
Although the co-axial package has a great advantage in the price thereof compared to the butterfly package, the space for installing the components is quite small and restricts the number of lead pins that can electrically communicate with the outside of the package. The number of lead pins is restricted to from 4 to 6 at most. While, the standard ROSA requires 5 lead pins at least for the power supply of the pre-amplifier in the ROSA, the ground, the signals complementary to each other output from the pre-amplifier, and the bias voltage to the APD. This example assumes that the grounds for the preamplifier and that for the APD are common, but the grounds may be independent to each other depending on the application which requires an additional lead pin.
A standard ROSA has an outer shape with a diameter of 4 mm to 5.6 mm, which restricts the number of lead pins to be from 4 to 6 as already mentioned. Furthermore, the ROSA is necessary to install a sub-mount to mount the APD and capacitors to bypass the bias voltage to the APD. Thus, it may be said that the co-axial package has no space to add the thermistor therein.
FIG. 4 is a plan view of the conventional ROSA. The APD 14 is mounted in a center portion of the stem 2 through the die-capacitor 9. The APD 14 is mounted on the upper electrode 10 of the die-capacitor 9. In the immediate side of the die-capacitor 9 is directly mounted with a pre-amplifier 25 on the stem 2, and other die-capacitors, 17 and 18 are mounted on the stem 2 in the other side of the pre-amplifier 25 with respect to the APD 14. These die-capacitors operate as a bypassing capacitor for the power supply of the pre-amplifier 25 and for the bias voltage to the APD 14, respectively. These components are installed within an area surrounded by four lead pins, 5 to 8. The ground lead pin is directly attached to the surface opposite to the surface illustrated in FIG. 4 and the ground lead does not pass through the stem 2. Although this ROSA has the stem whose diameter is about 5 mm, recent requests are to make the diameter thereof as small as possible.
Thus, five lead pins are preserved for the signals, the bias supply, and the ground, and 4 lead pins except for the ground lead pin is necessary to pass through the stem. When the components are installed within the area surrounded by these 4 lead pins on the upper surface of the stem, it may leave almost no space to add another lead pin to transmit the signal relating to the temperature of the APD.
Moreover, two types of thermistor are available in the market, that is, a chip thermistor and a die thermistor. The former device has a larger size compared to that of the latter device and two electrodes of the former device are necessary to be soldered to the substrate. While, the latter device, the die thermistor, has two electrodes in the top and bottom surfaces thereof, in which only the bottom electrode is necessary to be soldered, while, the top electrode may be wire-bonded. However, the die thermistor is generally expensive compared to the chip thermistor.